1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus in which an electrostatic latent image is formed on a latent image bearing member such as a photosensitive member with electroconductive toner.
2. Description of the Prior Art
Popularization of various image forming apparatus such as electrophotographic copiers and electrostatic recorders has induced a danger of illegal copying of private documents which the author does not wish to be copied, or official documents or banknotes.
A conventional countermeasure against such undesired copying consists of using, in the paper of the original document, watermarks or the like which do not appear or appear darker in the copy to facilitate the distinction of the copies from the originals. In such method, however, the paper to be used as the original is of a limited quality and is inevitably expensive. Besides such paper should not be easily available in order to prevent the use of such paper as the copying sheet.
Another countermeasure consists of marking an identification image in the copy of the original image. Such marking can be achieved by the use of plural exposure systems respectively for the original image and the identification image, or by conducting plural image forming cycles to superpose the identification image with the original image. In such method it is quite easy to form a black identification image in a white area of the copy or to form a white identification image in a black area of the copy. Such method is however not necessarily effective in case the original image does not have a determined pattern, since an identification image of a color which is the same as that of the copied image, or a white identification image on a white area is not easily recognizable.